Solid electrolytic capacitor and production method thereof, and conductive polymer polymerizing oxidizing agent solution.
1. Field of the Invention
The present invention relates to a solid electrolytic capacitor of a wound-roll type having an electrically conductive polymer as the electrolyte and a method of fabricating the same. Also, it relates to an oxidizing agent solution for polymerizing an electrically conductive polymer which is used as the electrolyte in a solid-electrolytic capacitor.
2. Background of the Invention
As a variety of electronic devices are now adapted for operating at higher frequencies, large capacitance type electrolytic capacitors are required which have favorable levels of impedance over the higher frequencies. For diminishing the impedance at higher frequencies, a solid electrolytic capacitor is introduced which employs as the electrolyte a highly electrical conductive material such as a tetracyanoquino-dimethane complex salt (referred to as TCNQ hereinafter) or any other conductive polymer. Also, for satisfying the demand of large capacitance, a wound-roll type of the solid electrolytic capacitor (having a positive foil and a negative foil separated by a separator and wound together in a roll) is proposed which is highly feasible to increase the capacitance as compared with a common electrode foil-layer type capacitor, while employing TCNQ or a conductive polymer as the electrolyte.
For use with such wound-roll type solid electrolytic capacitors, a number of methods for producing a highly conductive polymer have been developed. Among the most common methods for generating a solid electrolyte layer are electrolytic or chemical polymerization of a heterocyclic monomer solution and ah oxidizing agent solution alternately and electrolytic or chemical polymerization of a mixture of oxidizing agent solution and monomer solution for conductive polymer.
Typical examples of the heterocyclic monomer are pyrrole, thiophene, ethylene-dioxythiophene, aniline, and their derivatives. Also, examples of the oxidizing agent solution are alcohol solutions (methanol, ethanol, isopropyl alcohol, n-butanol, and ethylene glycol, etc.) containing p-toluene-sulfonic acid ferric salt, dodecyl-benzene-sulfonic acid ferric salt, naphthalene-sulfonic acid ferric salt, triisopropyl-naphthalene-sulfonic acid ferric salt, and long-chain alphatic sulfonic acid ferric salt. However, successful techniques have hardly been proposed for eliminating the effect of impurities contained in an oxidizing agent solution or a heterocyclic monomer used for chemically polymerizing a conductive polymer. Some conventional methods can simply specify the concentration of an oxidizing agent in an oxidizing agent solution.
It is known to use a separator for avoiding direct contact between the positive electrode foil and the negative electrode foil in a wound-roll type capacitor. The separator in a conventional electrolytic capacitor of a liquid electrolyte type is commonly an electrolyte-filled paper made of Manila hemp or Kraft paper.
In a wound-roll type solid electrolytic capacitor having an electrically conductive polymer as the electrolyte, an electrolyte-filled paper made of glass fiber based unwoven fabric, Manila hemp, or Kraft paper is rolled and then carbonized by baking for use as the separator (referred to as a carbonized paper hereinafter). A particular solid electrolytic capacitor is disclosed in Japanese Patent Laid-open Publication (Heisei)10-340829 where the separator is made of a synthetic unwoven fabric filled with Vinylon (a resin based on polyvinyl alcohol) or a composite unwoven fabric filled with Vinylon as a main component and other resin materials.
Also, uniquely proposed is a method of fabricating a wound-roll type aluminum electrolytic capacitor having TCNQ as the electrolyte which includes heating TCNQ to a temperature higher than its melting point to impregnate a capacitor element with melted TCNQ (comprising substantially 100% of a major component which contributes to the higher electrical conductivity) and then cooling down the capacitor element to yield a solid electrolyte layer of TCNQ between the positive electrode foil and the negative electrode foil. A related technique is provided for applying a dielectric oxide film, 2 to 5 volts, on the negative electrode foil in order to avoid inverse impression of voltage.
On the other hand, some techniques for improving a common aluminum electrolytic capacitor using a liquid electrolyte are disclosed in Japanese Patent Laid-open Publications (Showa)60-1826, (Heisei)1-304720, and (Heisei)9-186054 where providing a titanium layer or titanium nitride layer on the negative electrode foil made of a conductive metallic material such as aluminum to increase the static capacitance and prevent the electrolyte from being leaked out by the effect of electrochemical reaction over the negative electrode foil.
It is also known that solid electrolytic capacitors having a conductive polymer as the solid electrolyte, unlike common liquid electrolytic capacitors having a liquid electrolyte under an evaporating pressure, never contain a component which can easily evaporated at a higher temperature atmosphere (actually 200xc2x0 C. or more) during the soldering process for surface mounting electronic devices on a printed circuit board and eliminates substantially an unwanted event of pressure increase in the shell, hence minimizing undesired effects such as expansion of the shell or injury of the sealing members and probably being favorable for use in the surface mounting.
However, such solid electrolytic capacitors having a conductive polymer as the solid electrolyte have been developed much later than popular aluminum electrolytic capacitors having a liquid electrolyte and their techniques for surface mounting fail to be successfully practiced.
One of critical disadvantages of the solid electrolytic capacitors having a highly conductive TCNQ or polymer as the solid electrolyte is that the solid electrolyte has no ionic conductivity hence disabling to repair any damaged portion of the dielectric oxide film, allowing a higher rate of the leakage current, and making frequent occasions of short-circuit during the aging process.
Also, when a glass fiber based unwoven fabric is used as the separator in a wound-roll type capacitor, its rolled form is generally low in the physical strength and its fracture may injure the dielectric oxide film thus causing current leakage or short-circuit particularly during the aging process. The glass fiber unwoven fabric has a disadvantage that its needle-like fiber pieces separated during the cutting or rolling may be scattered in all directions thus damaging the working environment.
The carbonized electrolyte-filled paper assists the TCNQ or conductive polymer to diminish the impedance at high frequencies only when the capacitor element is heated to as a high temperature as over 250xc2x0 C. The heating up may however injure the dielectric oxide film and increase the leakage current. Also, as the platings (commonly of tin/lead solder) on the leads of the capacitor element are oxidized by the heating, their soldering affinity on the leads of a finished capacitor may significantly be declined. For improvement, the use of silver plated leads which are high in the resistance to oxidation shall be required, resulting in the cost up.
The composite fabric consisting mainly of Vinylon-based unwoven fabric and Vinylon-based resin is lower in the tension strength than the electrolyte-filled paper and may easily be injured during the rolling to form a capacitor element, hence causing short-circuit during the aging process. Also, as an adhesive is used for bonding resin fibers to one another to have a sheet, it may interrupt the application of the conductor polymer to the separator, thus discouraging the fabrication of a solid electrolytic capacitor having a lower impedance at high frequencies. The Vinylon-based resin is low in the resistance to heat and may possibly be decomposed during the use of the solid electrolytic capacitor under a higher temperature condition or during the high-temperature reflow process for soldering. This generates a considerable amount of gas and increases the inner pressure, hence injuring the seals and finally impairing the electric characteristics of the solid electrolytic capacitor.
It is admitted that the solid electrolytic capacitor having TCNQ or conductive polymer as the solid electrolyte, unlike the liquid electrolytic capacitor having a liquid electrolyte (an electrolytic solution), never have any component in its element which may easily be evaporated under a high temperature atmosphere (practically at 200xc2x0 C. or more) during the soldering for surface mounting electronic devices on a printed circuit board. This will minimize increase of the pressure in the shell and eliminate undesired expansion of the shell or damage to the seal members, permitting the solid electrolytic capacitor to be surface mounted with much ease. It is however found through a series of experiments by us, the inventors, that as the solid electrolyte positively absorbs water in the air, the absorbed water trapped in the capacitor element during the previous step shall be evaporated under the high temperature atmosphere in the surface mounting process and the surface mounting of the solid electrolytic capacitor will be declined in the efficiency as equal to that of the liquid electrolytic capacitor.
More specifically, as the glass fiber unwoven fabric serving as the separator contains highly hydrophilic silica as a main component, it can absorb more water in the air than the resin separator and allows the capacitor element to hold a higher volume of water, thus discouraging the surface mounting process.
The carbonized electrolyte-filled paper is made of a cellulose which also has a higher wetting affinity and can absorb more water in the air than the resin separator. This permits the capacitor element filled with the solid electrolyte to hold a higher volume of water, thus discouraging the surface mounting process.
Meanwhile, the conductive polymer used as the solid electrolyte may be polypyrrole or polyethylene-dioxythiophene prepared by chemically polymerizing ethylene-dioxythiophene with an appropriate oxidizing agent. It is however practically difficult to coat the carbonized paper, glass fiber based fabric, or Vinylon- or polypropylene-based unwoven fabric with the above mentioned conductive polymer. Accordingly, the conductive polymer is likely detached off from the separator by the effect of thermal stress or the like, hence increasing the impedance and declining the capacitance drawing rate. As a result, the overall size per storage capacitance of the solid electrolytic capacitor will be greater than that of any liquid electrolytic capacitor.
The wound-roll type solid electrolytic capacitor is significantly lower in the static capacitance drawing rate on the negative electrode foil than on the positive electrode foil. While the static capacitance drawing rate on the positive electrode foil is sufficient, the overall static capacitance of the wound-roll type electrolytic capacitor is a serial sum of the static capacitance drawn from the positive electrode foil and the static capacitance drawn from the negative electrode foil and will thus be declined to as a small value as 40 to 50%. Accordingly, the overall size per capacitance of the solid electrolytic capacitor will be greater than that of any liquid electrolytic capacitor.
For improvement, a technique is used for impregnating the capacitor element with a polymeric solution which contains a heterocyclic monomer such as ethylene-dioxythiophene, an oxidizing agent such as p-toluene-sulfonic acid ferric salt, and a solvent (water or alcohol such as n-butanol) to develop a large amount of the conductive polymer adjacent to the surfaces of the electrode foils through chemical polymerization. For implementation of the technique, it is desired to have the separator arranged high in the liquid retention and low in the physical density. The lower the density, the higher the occurrence of short-circuit will increase.
When the solid electrolytic capacitor has a rated voltage of higher than 16 V and provided with its separator increased in the density to minimize the short-circuit, its withstand voltage will be improved and simultaneously the bulk density of the separator will increase. This reduces the polymeric solution retained adjacent to the electrode foils and hardly generates a desired amount of the conductive polymer as the solid electrolyte for drawing a given static capacitance.
It is hence understood from the foregoing reasons that the capacitor element of a wound-roll type is hardly impregnated with the conductive polymer uniformly and sufficiently. In particular, the electrical characteristics of polyethylene-dioxythiophene prepared by polymerizing ethylene-dioxythiophene (including the static capacitance determined by the coating adhesivity of the conductive polymer to the dielectric oxide film and the impedance at higher frequencies determined by the filled amount of the conductive polymer) may be varied depending on the property of an oxidizing agent used (more specifically, a difference of the oxidizing agent solution or the heterocyclic monomer between supply lots), the polymerizing conditions, and the time elapsed from the preparation of an oxidizing agent to the polymerization of a material monomer for the conductive polymer.
It is hence an object of the present invention to provide a solid electrolytic capacitor which eliminates the above disadvantages, minimizes the current leakage, and has a large capacitance and a higher resistance to heat to be desirable as a surface-mount electronic device and a method of fabricating the same. It is another object of the present invention to provide an oxidizing agent solution for preparing a conductive polymer to fabricate the above mentioned solid electrolytic capacitor at a higher yielding rate as well as a-higher level of stableness.
The present invention has the following features.
(1) A layer containing an electroconductive polymer and a less conductive polymer is disposed on a dielectric oxide film on the positive electrode made of a valve metal.
(2) A method of producing a solid electrolytic capacitor of a wound-roll type having the above (1) construction comprises-the steps of: immersing a capacitor element fabricated by sandwiching a separator between a positive electrode foil coated with a dielectric oxide film and a negative electrode foil and rolling together into a solution which contains at least an electroconductive polymer and a less conductive polymer (a non-conductive. polymer); heating the capacitor element to evaporate a solvent in the solution and thus develop on the dielectric oxide film on the positive electrode foil a layer containing the electroconductive polymer and the less conductive polymer; and immersing the capacitor element in admixture solution which contains at least an oxidizing solution for polymerizing a conductive polymer and a heterocyclic monomer, or immersing the capacitor element in a solution which contains the oxidizing solution for polymerizing a conductive polymer and then in another solution which contains the heterocyclic monomer, or immersing the capacitor element in a solution which contains the heterocyclic monomer and then in another solution which contains the oxidizing solution for polymerizing a conductive polymer to generate a solid electrolyte made of the conductive polymer between the positive electrode foil and the negative electrode foil.
(3) An unwoven fabric separator made by span bonding and/or wet processing a resin based material is sandwiched between a positive electrode foil coated with a dielectric oxide film and a negative electrode foil and rolled together to form a capacitor element while a solid electrolyte containing the conductive polymer is provided between the positive electrode foil and the negative electrode foil in the capacitor element. Particularly, the separator may be an unwoven fabric based on a polyester resin which is polyethylene-terephthalate or its derivative.
(4) A separator (an unwoven separator based on a resin such as polyethylene-terephthalate) is sandwiched between a positive electrode foil coated with a dielectric oxide film and a negative electrode foil coated with a dielectric oxide film having 0.8 to 10 V of withstand voltage and rolled together to fabricate a capacitor element while a solid electrolyte between the positive and negative electrode foils in the capacitor element is an electrically conductive polymer.
(5) A separator (an unwoven separator based on a resin such as polyethylene-terephthalate) is sandwiched between a positive electrode foil coated with a dielectric oxide film and a negative electrode foil coated with a layer of a metallic material selected from titanium, zirconium, and hafnium or its compound or a carbon material and rolled together to fabricate a capacitor element while a solid electrolyte between the positive and negative electrode foils in the capacitor element is an electrically conductive polymer.
(6) A solid electrolytic capacitor comprises: a capacitor element fabricated by sandwiching and rolling a separator between a positive electrode foil coated with a dielectric oxide film and a negative electrode foil, having a solid electrolyte layer containing a conductive polymer between the positive electrode foil and the negative electrode foil, and arranged to limit the content of water to not higher than 1% by weight based on its weight; an outer case of a bottomed cylindrical shape made of a metallic material in which the capacitor element is enclosed; and a sealant arranged containing a polymer for sealing off the opening of the outer case.
(7) A solid electrolytic capacitor comprises: a capacitor element fabricated by sandwiching and rolling a separator between a positive electrode foil coated with a dielectric oxide film and a negative electrode foil and having a solid electrolyte layer containing a conductive polymer between the positive electrode foil and the negative electrode foil; an outer case of a bottomed cylindrical shape made of a metallic material in which the capacitor element is enclosed; and a sealant made of a peroxide vulcanized and/or resin vulcanized butyl rubber, which has a resiliency of not smaller than 450 N/cm2 at 250xc2x0 C., for sealing off the opening of the outer case.
(8) Alternatively, an oxidizing solution may be used which has a ferric salt of aliphatic and/or aromatic sulfonic acid dissolved in an alcohol solvent and of which the weight ratio of bivalent iron to trivalent iron is not higher than 0.02.
(9) The solid electrolyte may be a conductive polymer made by chemically polymerizing a heterocyclic monomer with the use of an oxidizing solution which has a ferric salt of aliphatic and/or aromatic sulfonic acid dissolved in an alcohol solvent and of which the molar ratio of the aliphatic and/or aromatic sulfonic acid to trivalent iron ranges from 3.0 to 3.5.
(10) The solid electrolyte may be a conductive polymer made by chemically polymerizing a heterocyclic monomer using an oxidizing solution in which the weight ratio of bivalent iron to trivalent iron is not higher than 0.02 and the molar ratio of aliphatic and/or aromatic sulfonic acid to trivalent iron ranges from 3.0 to 3.5.
(11) The solid electrolyte may be a conductive polymer made by chemically polymerizing a heterocyclic monomer which contains not higher than 0.8% of remaining basic organic solvent as impurities.
(12) A method of producing the solid electrolytic capacitor comprises the steps of: sandwiching and rolling a separator (an unwoven separator based on a resin such as polyethylene-terephthalate) between a positive electrode coated with a dielectric oxide film and a negative electrode of etched aluminum to form a capacitor element; and yielding a solid electrolyte containing a conductive polymer (namely polyethylene-dioxythiophene) between the positive electrode foil and the negative electrode foil with the use of a combination of an oxidizing solution for polymerizing a conductive polymer which has a ferric salt of aliphatic and/or aromatic sulfonic acid (namely p-toluene ferric sulfonate) dissolved in an alcohol solvent and in which the weight ratio of bivalent iron to trivalent iron is not higher than 0.02 and the molar ratio of the aliphatic and/or aromatic sulfonic acid to trivalent iron ranges from 3.0 to 3.5 and a heterocyclic monomer (namely ethylene-dioxythiophene) which contains not higher than 0.8% of remaining basic organic solvent as impurities. Alternatively, this method may be combined with the method depicted in the above paragraph (2).
The solid electrolytic capacitor according to the present invention can hence be minimized in the leakage current and increased in the capacitance while having a higher resistance to heat, being favorable as a surface-mount type device. Also, the oxidizing solution for polymerizing an electrically conductive polymer and the heterocyclic monomer for a conductive polymer according to the present invention can contribute to production of a solid electrolytic capacitor having improved electric characteristics at a higher rate of yield and a higher stability.